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Telescope Magnification Calculator

Work out telescope magnification instantly with clear inputs, formula shown and shareable results.

About Telescope Magnification Calculator

Magnification is objective focal length divided by eyepiece focal length: a 1200 mm scope with a 10 mm eyepiece gives 120x. The exit pupil, aperture divided by magnification, is 1.25 mm here - comfortable for planetary work. Maximum useful magnification is roughly twice the aperture in millimetres, beyond which the image only grows blurrier.

Formulas

Magnification

M = objective focal length / eyepiece focal length

Exit pupil

exit pupil = aperture / magnification

Worked example

With the default values below, Telescope Magnification Calculator returns Magnification of 120.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Objective focal lengthminimum 1 mm
1,200 mm
Eyepiece focal lengthminimum 0 mm
10 mm
Apertureminimum 1 mm
150 mm

Results

Magnification
120.0 x
Exit Pupil Diameter
1.25 mm
Maximum Useful Magnification
300 x
Focal Ratio
8.00

Step by step

  1. 1Values used
  2. 2Magnification
  3. 3Exit pupil
  4. 4Magnification= 120.0 x
  5. 5Exit Pupil Diameter= 1.25 mm
  6. 6Maximum Useful Magnification= 300 x
  7. 7Focal Ratio= 8.00

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is there a maximum useful magnification?

Because diffraction sets the resolution. Past about 2x the aperture in millimetres you magnify blur, not detail.

What exit pupil should I aim for?

About 2-3 mm for general viewing, 5-7 mm for faint deep-sky objects and under 1 mm only for very high power planetary work.

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